The present invention is directed to an apparatus and method for producing an analog output proportional to the rotational speed of a shaft.
An analog measure of shaft rotational speed is useful in its raw form and also as an analog input for calculating other mechanical system parameters such as horsepower. In marine applications such analog system data is particularly useful in realizing maximum efficiency from a new generation of ships having variable pitch propellers. To obtain shaft horsepower it is necessary to know the values of torque and shaft speed. Apparatus for producing an analog output proportional to shaft torque is disclosed in U.S. Pat. No. 3,850,030, assigned to the present assignee.
Shaft speed measuring systems employing a direct mechanical connection between a shaft and monitoring gauge are expensive to install and calibrate, subject to constant wear, and are dependent on the accuracy of the mechanical gauge.
Electronic systems of measurement face the problem of the wide range of operating speeds encountered for ship driveshafts. Analog systems which measure partial or full shaft rotations or measure rotation time intervals are unworkable when the measured speeds range from less than one to over 100 r.p.m. Although digital computation devices will accurately handle such a wide range of values, analog devices do not due to inaccuracies in the analog multipliers used to calculate r.p.m.
Multiplying digital-to-analog converters (MDACs) would offer more accuracy in producing analog output values from digitally timed shaft rotation intervals, but they have a limited useful input range. A ten-bit MDAC can accept digital inputs between decimal values of 1 and 1023 only, and even this limited range cannot be fully utilized. Digital inputs at the lower end of the range contain too few significant bits to provide the high accuracy analog r.p.m. output desired. The operating range of a ship's driveshaft greatly exceeds the useful input range of such a converter.
Multiplying digital-to-analog converters having capacities exceeding ten bits are not readily available. Moreover, even converters accepting 11, 12 or more bits will fail to handle adequately the full range of speeds encountered in a ship's driveshaft.